1. Field of the Invention
The invention relates to a method for realizing an effective value of a quantity to be varied in an electrical load connected to a multi-phase switchable DC/AC frequency convertor comprising a plurality of controllable switches.
2. Description of Related Art
An electrical load within the context of the present invention is e.g. a controllable electrical drive unit consisting of a multi-phase (usually three-phase) electric motor, whose windings are connected to the frequency convertor. The frequency convertor is fed with direct current, which direct current is converted into a controllable alternating current by suitably turning the switches of the frequency convertor on and off. Consequently, the frequency convertor converts a direct current voltage (DC) into an alternating current voltage (AC) having an adjustable amplitude and frequency. In English professional literature, such frequency convertors are also indicated by the term “invertor”.
In practice three main groups of frequency convertors can be distinguished, the so-called voltage source convertors, indicated by the acronym VSI (“Voltage Source Invertor”) in English professional literature, current source convertors, indicated by the acronym CSI (“Current Source Invertor”) in English professional literature, and the so-called matrix convertors. In practice, VSI's are used by far the most.
Other examples of electrical loads within the context of the present invention are e.g. transformers, power resistors and the like, for alternating current such loads have a “high-frequency impedance” sufficiently higher than zero for control by a VSI, or a “high-frequency impedance” sufficiently close to zero for control by a CSI.
In its simplest form, a three-phase VSI consists of six unidirectional controllable switches, usually IBGT- or GT0-type semiconductor switches, and one or more capacitors as a DC interstage circuit buffer. In its simplest form, a three-phase CSI likewise comprises six unidirectional controllable switches, but on the other hand it comprises an induction coil as the DC interstage circuit buffer. A matrix convertor does not comprise an interstage circuit, but in its simplest three-phase form it comprises nine bidirectional controllable switches.
The advantage of the VSI is that it can be fed from a passive rectifier, whereas a CSI needs an invertor as the connection to the power-supplying electricity grid.
A CSI by definition behaves like a current source at its output terminals, whereas the VSI behaves like a voltage source and the matrix convertor exhibits the input impedance.
Because of its practical simplicity, a VSI is generally used in practice, for example for realizing a desired motor current in an electric motor, which VSI is controlled in such a manner that the desired motor current will flow in the electric motor. In order to enable proper adjustment of the force or the torque of the electric motor that is being generated, quick and accurate adjustments of the current through the motor must be possible. When a VSI is used, the voltage delivered by the VSI will have to be varied. The use of a desired voltage value related to the motor current to be realized has a delaying effect on the speed at which the motor current can be adjusted, however.
In practice, the dynamics of a current-controlled VSI strongly depend on the parameters and the working point of the connected electric motor. That is, in those cases in which the motor parameters are subject to change, or in the case of strongly varying loads, a current-controlled VSI functions anything but optimally. An example of this is a large linear electric motor whose stator is switched in portions, so that the inductivity will change in steps.